Correctly lapping and fixing weatherboards is one of the most important steps in keeping your home weathertight. Done right, a weatherboard cladding system will shed water reliably for decades; done wrong, it can lead to costly moisture damage and potential breaches of the NZ Building Code. Here's what you need to know before you pick up a hammer.
Understanding the NZ Building Code Requirements
Weatherboard cladding must comply with NZ Building Code Clause E2 (External Moisture), which sets out performance requirements for keeping water out of the building envelope. Most residential timber-framed homes also fall under NZS 3604, the standard for light timber framing. Your cladding system — including the lap, fixings, and any underlays — must meet these requirements.
For most projects involving new cladding on an existing home, a building consent may be required. Schedule 1 of the Building Act lists some exemptions (such as like-for-like repairs of small areas), but the rules are nuanced. Always check with your local council before starting work to confirm whether consent is needed.
For detailed technical guidance, BRANZ publishes excellent resources on cladding installation best practice that are well worth reading before you begin.
Choosing Your Weatherboards
Common weatherboard profiles in NZ include:
- Bevel-back (colonial) – tapered boards, the most traditional NZ style
- Rusticated – rebated boards that create a shadow line
- Cavity-back – boards with a moulded back groove to assist drainage
- Fibre cement – products like those from James Hardie that offer excellent durability and low maintenance
Timber species commonly used include radiata pine (pre-primed), cedar, and macrocarpa. Always use pre-primed or pre-treated boards where possible, and ensure any cut ends are sealed with primer before installation.
Setting Up: Underlay and Cavity
Before fixing any boards, make sure your wall underlay (building wrap) is correctly installed, lapped, and taped. Modern best practice — and E2 requirements for many wall types — calls for a drained and ventilated cavity between the underlay and the cladding. This is typically achieved using cavity battens (usually 20 mm thick) fixed over the studs through the underlay into the framing.
The cavity allows any water that gets behind the cladding to drain out at the bottom rather than sitting against the framing. Check MBIE Building Performance for the specific E2 requirements that apply to your wall type and exposure zone.
Calculating and Setting the Lap
The lap is the amount one board overlaps the one below it. Getting this right is critical for weathertightness.
- For most bevel-back weatherboards, the minimum lap is 25 mm, but many installers use 30–35 mm for better weather resistance, especially in exposed locations.
- Manufacturer instructions always take precedence — check the specific product data sheet.
- Use a story pole (a length of timber marked with your board spacing) to keep courses consistent from one end of the wall to the other.
- Start from the bottom of the wall and work upward, using a starter strip (a thin packing piece) at the base to kick the first board out at the correct angle.
| Board width | Typical face exposure | Minimum lap |
|---|---|---|
| 150 mm | ~120 mm | 25–30 mm |
| 180 mm | ~150 mm | 25–30 mm |
| 200 mm | ~165 mm | 30–35 mm |
Face exposure and lap will vary by profile and manufacturer — always confirm with the product specification.
Fixing Weatherboards Correctly
Proper fixing is just as important as the lap. Key rules:
- Use the right nails. For timber weatherboards, use hot-dipped galvanised (HDG) nails — at minimum 2.8 mm diameter and long enough to penetrate at least 40 mm into the framing (stud or cavity batten). Stainless steel nails are preferred in coastal or high-corrosion areas.
- Nail through the face, just above the top of the board below — do not nail through both boards at the lap, as this prevents the boards from moving with seasonal timber expansion and can cause splitting.
- Fix into every stud (typically at 600 mm centres for NZS 3604 framing).
- For fibre cement boards, follow the manufacturer's fixing schedule exactly — James Hardie provides detailed installation guides for their products.
- Pre-drill near board ends to prevent splitting, especially with timber boards.
Sealing Joints, Ends, and Penetrations
All butt joints between boards should be avoided where possible by using full-length boards. Where joints are unavoidable:
- Locate them over a stud.
- Back-prime all cut ends thoroughly before fixing.
- Use a backing rod and paintable exterior sealant (such as a product from Sika) to seal the joint, or use a purpose-made joiner moulding.
All penetrations (pipes, cables, taps) must be properly flashed and sealed. Window and door junctions are particularly critical — these must be correctly flashed in accordance with E2 requirements.
Priming and Painting
All bare timber must be back-primed (primed on the back face and edges) before installation to reduce moisture uptake. Once fixed, apply a full exterior paint system as soon as possible — ideally within two weeks of installation. Quality exterior paint systems from Resene, Dulux, or Wattyl will provide long-lasting protection when applied correctly.
When to Call a Professional
While an experienced DIYer can tackle weatherboard repairs or small replacement sections, larger cladding projects are best handled by a Licensed Building Practitioner (LBP). Under the Building Act, certain building work on homes must be carried out or supervised by an LBP — and cladding is one of the restricted building work categories when it forms part of the weathertight envelope.
If your project requires a building consent, the cladding work will need to be done by or under the supervision of an LBP. You can verify a practitioner's licence at Licensed Building Practitioners. To find a qualified local carpenter or cladding specialist, search tradedirectory.co.nz.
